Room: ePoster Forums
Purpose: Considering skin sparing is essential in passive scattering proton therapy (PSPT), in which a small number of fields can cause high entrance dose. In vivo skin dosimetry (IVSD) is required to identify actual skin dose; however, the methodology of IVSD in PSPT remains unestablished to date. Recently developed thermoluminescence sheet-type dosimeter (TLD sheet) is particularly suitable for applications in PSPT owing to their water-equivalent properties, ease of use, and ability to adjust size. Therefore, this study aimed to investigate the utility of TLD sheet for IVSD in PSPT.
Methods: TLD sheet (TOYO Medic Ltd., Tokyo, Japan) contains lithium triborate as the main component, and its effective atomic number is 7.3. To assess the fundamental characteristics of TLD sheet such as dose linearity, uniformity, and energy dependence, we irradiated 150-MeV modulated and unmodulated proton beams onto TLD sheet positioned perpendicular and vertical to the beam axis in a water phantom. Moreover, to assess the actual range shift of the proton beam because of the insertion of the TLD sheet in the beamline, we performed proton depthâ€“dose measurement to obtain water-equivalent thickness (WET) with or without TLD sheet attached to the entrance wall of the water phantom.
Results: We confirmed good uniformity and linearity between dose and TLD sheet signal. The depthâ€“dose curve measured with a TLD sheet well reproduced the results obtained with an ionization chamber, except for the Bragg peak and distal region of a spread-out Bragg peak. Moreover, we found a quenching effect of up to 20% in these regions. The measured WET of a single sheet was 0.2 mm without the protective sheet.
Conclusion: Despite some limitations and needs for improvement, the impact on range error and energy dependences is almost negligible in IVSD; thus, novel TLD sheet could be used as IVSD tools in PSPT.